Dead zones a coastal threat

A dead zone in San Diego Bay is part of a new national assessment of coastal waters suffering from low levels of oxygen. It shows areas of hypoxia have increased rapidly over that past 30 years.
— Earnie Grafton

A dead zone in San Diego Bay is part of a new national assessment of coastal waters suffering from low levels of oxygen. It shows areas of hypoxia have increased rapidly over that past 30 years.
— Earnie Grafton

Dead zones were detected in nearly half of the 647 waterways assessed, including the Gulf of Mexico, home to one of the largest such zones in the world. The impact of the BP Deepwater Horizon oil spill on oxygen levels in the gulf was not assessed because it occurred after the report was written.

On the West Coast, federal researchers found a sixfold increase in the number of dead zones over the past 20 years, with 37 areas now suffering from low oxygen. A region off the coast of Oregon and Washington has become the second-largest seasonal hypoxic region in the United States and third largest in the world.

The trends highlighted in Friday’s report are part of a growing global problem, said Tony Koslow, who studies low-oxygen areas at the Scripps Institution of Oceanography, part of the University of California San Diego in La Jolla.

He said researchers around the Pacific Rim have noticed decreasing oxygen levels over the past decade.

“This is a large phenomenon not due to nutrient outflows” from land, said Koslow. “This big question is, ‘Is this due to climate change?’ ”

As the top oxygen-rich layer of the ocean warms, Koslow said it mixes less with the colder, oxygen-poor layers of the deep. Global climate models predict that the oxygen levels in deep oceans will decline 20 to 40 percent the next century.

“There are substantial ecosystem concerns,” Koslow said. “A number of species that live in the deep ocean are very sensitive to changes in oxygen levels. These species — although they are not of commercial interest — are prey to squid, fish, marine mammals and seabirds so changes in oxygen will have repercussions throughout the food web.”

Friday’s report said work to study and control pollutants are advancing but management efforts to stem the tide of hypoxia “have not made significant headway” in part due to increased development and population growth in coastal watersheds.

“If current practices are continued, the expansion of hypoxia in coastal waters will continue and increase in severity, leading to further impacts on marine habitats, living resources, economies, and coastal communities,” the report’s authors said.